1. Field of the Invention
The present invention relates to compositions for heavy brines systems for use as drill-in and completion fluids.
2. Description of Relevant Art
Drill-in fluids are drilling fluids used in drilling through a hydrocarbon producing zone (also called a pay zone) of a hydrocarbon bearing subterranean formation and completion fluids are fluids used in completing or recompleting or working over a well. Completion operations normally include perforating the casing and setting the tubing and pumps prior to, and to facilitate, initiation of production in hydrocarbon recovery operations. The various functions of drill-in, completion and workover fluids include controlling well pressure, preventing the well from blowing out during completion or workover, and preventing the collapse of the well casing due to excessive pressure build-up. The fluid is meant to help control a well without damaging the producing formation or completion components. Specific completion fluid systems are selected to optimize the well completion operation in accordance with the characteristics of a particular geological formation.
Choosing the right completion fluid is important because inappropriate fluids can have a significant impact on a project, not only during completion operations and well production startup, but also throughout the well's productive life. Experience has shown that some completion practices that work well in one location, such as, for example, the shelf of the Gulf of Mexico, may not work well or transfer directly to a different location, such as, for example, the deepwater environment. Deepwater completions can be more challenging because of the combination of younger formations, colder seafloor temperatures, greater hydrostatic pressures and interaction with subsea systems and control fluids.
Seldom is a regular drilling fluid suitable for completion operations due to its solids content, pH and ionic composition. Drill-in fluids can, in some cases be suitable for both drilling and completion work. Fluids can contain suspended solid matter consisting of particles of many different sizes. Some suspended material will be large enough and heavy enough to settle rapidly to the bottom of a container if a liquid sample is left to stand (the settable solids). Very small particles will settle only very slowly or not at all if the sample is regularly agitated or the particles are colloidal. These small solid particles cause the liquid to appear turbid (i.e., cloudy or hazy). The potential of particle invasion and/or filter cake buildup to damage a formation by reducing permeability in the producing zone has been recognized for many years. If permeability gets damaged, it cannot be 100 percent restored by any means. Loss in permeability means a decrease in anticipated production rates and ultimately in a decrease in production overall.
The importance of using clear completion and workover fluids to minimize formation damage is now well recognized and the use of clear heavy brines as completion fluids is now widespread. As used herein, “clear” or “clear and colorless” with respect to brine or completion fluids means that the fluid has an “NTU” (nephelometric turbidity unit) less than about 20. NTU is an American Petroleum Institute accepted unit related to the suspended solids in a brine (higher NTU=more suspended solids), based on how much light is scattered by a sample. The procedure for determining NTU is described in API RP 13J, “Testing of Heavy Brines,” incorporated herein by reference, and is a procedure well known to those of ordinary skill in the art. As used herein, “heavy” with respect to brine or completion fluids means that the fluid has a density at least above about 10 lb/gal and most typically in the range of about 10 lb/gal to about 20 lb/gal, as needed to compensate for high downhole subterranean pressures.
Most such heavy brines used by the oil and gas industry are calcium halide brines, particularly calcium chloride or calcium bromide brines, or formate brines. However, halide brines can cause structural failure in corrosion resistant alloys, and chloride and bromide brines in particular are known to cause pitting corrosion and stress corrosion cracking of corrosion resistant alloys if oxygen or carbon dioxide is present. Formate brines do not cause such corrosion and cracking but are more costly to purchase and have some solubility problems at high density. Thus, there is continued interest in economically priced heavy brines that are not highly corrosive and that do not readily cause formation damage for use as completion fluids.